Across the map of Toronto, there are several “Old” versions of major streets: Old Yonge Street, Old Leslie Street, et cetera. These are smaller and certainly older streets that predate yet still exist alongside their longer, newer counterparts.
How old are these “old” streets anyways? Why were they built as they were in the first place? Why were they replaced?

Source: Old Toronto Maps
Here are five examples of “Old” Toronto Streets and their histories:
1. Old Yonge Street
Year rerouted: 1835
When Yonge Street was laid out in the 1790s, it was not the continuous straight path we think of today. The sheer length of the street almost welcomed obstacles. At York Mills, the challenging topography around the West Don River caused it to divert east just south of York Mills Road. It curved north and back west to join the original course. In 1835, the street was realigned and straightened. It seems in the 1920s, Yonge Street was re-routed again slightly to the west to allow for better automobile navigation.

Source: Old Toronto Maps

Source: City of Toronto Archives.

Source: Globe & Mail Archives.
Today, the old, “orphaned” course remains as part of Mill Street and Old Yonge Street. Old Yonge’s narrow, curvy course in parts maintains a rural quality. While at one time Yonge and Old Yonge once connected at its north end, this connection is now a roundabout. Finally, because of its length in the province, there are other Old Yonge Streets in Thornhill and Aurora.

Source: Google Maps

Source: Google Maps.

2. Old Sheppard Avenue
Year rerouted: ~1934
Sheppard Avenue once existed in two separate sections on either side of the Scarborough-North York border. A traveller wishing to travel east or west through the two streets had to jog about 300 metres on Victoria Park to reach the other section. In 1934, the two roads were joined through a curving road running from just past Woodbine Avenue to the lower street in Scarborough. The move was the idea of Ontario Premier George S. Henry whose estate stood where the new Sheppard Avenue connection ran.

Source: City of Toronto Archives
Today, the orphaned North York section of the old road now exists as residential Old Sheppard, albeit with small parts removed around Highway 404.

Source: Google Maps

Source: Google Maps

More reading: Scenes From O’Sullivan’s Corners & Muirhead’s Corners
3. Old Lawrence Avenue
Year rerouted: ~1961
Lawrence Avenue is and was one of many streets which was impact by Toronto’s ravines. West of Victoria Park Avenue, Lawrence once took an interesting route across the East Don River Valley. Like Sheppard Avenue, there were two sections of the street: the Scarborough section which exists today and a North York section. The North York section jogged up Victoria Park over the Canadian Pacific Railway, ran briefly next to the track, and continued west for 1.5 kilometres. From here, it took a rather curvy route south down the East Don Valley, crossed the Don River via a bridge, and curved back north and west before continuing towards Don Mills Road. Presumably, this was easiest way in the 19th century to navigate the valley.

Source: Old Toronto Maps

Source: Toronto Public Library

Source: Toronto Public Library
In 1961, Lawrence Avenue was straightened with a road directly connecting Victoria Park and Woodcliff Place, curling northwest from Scarborough with several new bridges to accommodate the Don River and CPR.

Source: City of Toronto Archives.

Source: City of Toronto Archives.
Today, the orphaned old road exists as roughly as part of Roanoke Road and, more famously, a short access road to the East Don Trail named Old Lawrence. The remaining section west of the river along with the old bridge itself have been lost.

Source: Google Maps

Source: Google Maps

More reading: Scenes From East Don Trail
4. Old Leslie Street
Year rerouted: ~1968
Like Lawrence Avenue, Leslie Street’s course at one time also had to divert around the East Don River. Also of 19th-century origin, a traveller going north on Leslie had to turn west for a short distance and then northwest for about 500 metres to meet with Sheppard Avenue. There was then a jog east on Sheppard, which included a bridge over the river and finally a left turn to travel north again.

Source: Old Toronto Maps

Source: City of Toronto Archives

Source: City of Toronto Archives.
In the 1950s, with the construction of Highway 401, Leslie Street was altered to curve through the highway, but the course has otherwise remained the same. In 1968, the street was reconfigured again to join with Sheppard more directly. The Don River was also straightened and a new bridge was constructed which spanned the entirety of the new four-way intersection.

Source: City of Toronto Archives.
Today, the old course remains as Old Leslie Street, albeit a shorter version of the original route is available today to the public. It joins the new Leslie Street via Esther Shiner Drive. South of that street, there are City facilities. North of Esther Shiner, Old Leslie serves the Leslie Street TTC Station before it crosses over Sheppard via an overpass. It then curls back down to join the street (there is also a parking lot with an entrance to the East Don Parkland trail).

Source: Google Maps

Source: Google Maps

More reading: Scenes From East Don Parkland
5. Cummer Avenue
Year rerouted: ~1969
The original course of Cummer Avenue west of Leslie Street was an 1819 construction. The street was laid out as a side road from Yonge Street by the Cummer family to access their holdings (a mill and camp) near the East Don River. When it approached the valley, it curved down to roughly follow the river’s course. It crossed the river via a bridge and eventually the railway tracks at a level crossing. Finally, it terminated at Leslie Street.

Source: Old Toronto Maps

Source: City of Toronto Archives
By 1969, the street was rerouted to curve north away from the river (which looks to have been straightened around this time as well). The street passed through a new wider bridge over the Don River and then under a railway overpass before eventually becoming McNiccol Avenue at Leslie Street.

Source: City of Toronto Archives
The old, orphaned course still exists in parts. The curved section lives on as part of the East Don Parkland trail, although not all of it follows the old path. The old bridge is in situ as well. The trail travels east through the hydro corridor where it terminates at the railway tracks. On the other side, Old Cummer Go Station and a hundred-metre long Old Cummer Avenue hold the old name.

Source: Google Maps

Source: Google Maps

More reading: Scenes From East Don Parkland
Click here for the map below of “Old” Streets.

For more “Old” Streets, I created a sequel here.
One small possible correction. I don’t think Yonge Street itself was diverted to the west in the 20s, I think it’s referring to what is now Yonge Blvd. Most of that route still exists, however the northern section was incorporated into the 401 when it was being built.
You can actually see the pre-401 route in the 1950’s picture you have above. Avenue road connected into that road as well from the west. The “magnificent viaduct” spoken about in the news clipping was the Yonge Blvd viaduct, started in 1928, opened in 1929, and now makes up one of the four spans of the Hoggs Hollow bridge which carries the eastbound express lanes of the 401 over the ravine.
Here’s a picture from the city archives from 1950, someone actually penciled in the future avenue road interchange in red on the photo to give you an idea. The present day eastbound on-ramp used to be the main route of Avenue road which connected into Yonge Blvd, and then Yonge St. Probably also explains why Yonge was Hwy 11 and Avenue was 11a as it was an alternate route into the city from this now lost connection.
https://www.toronto.ca/ext/archives/s0012/fl1950/s0012_fl1950_it0008g.jpg
Here’s a great Mike Filey article on it as well: https://torontosun.com/news/local-news/the-way-we-were-tribute-to-90-year-old-hoggs-hollow-bridge