The tourists scored nine tries with Pontypridd's Gareth Wyatt claiming a hat-trick while fellow wing Dafydd James ran in two.
However, midway through the first half Wales had actually found themselves behind after Ontario's full-back Joe Pagano made the most of some weak tackling to score a try which Steve Pettigrew converted to add to his earlier penalty.
But the Welsh responded as Neal Boobyer ran in a try from the restart which Lee Jarvis converted to regain the lead at 15-10.
Wyatt and Jarvis ran in further tries to make it 25-10 at the interval, and Ontario were unable to break down the Welsh defence throughout the second half. Instead the tries flowed freely for the tourists.
The only downside for the Welsh was a dislocated shoulder for the Pontypridd prop Aled Griffiths, who only arrived in Canada on Sunday as a replacement for Christian Loader.
Ontario: Try Pagano; Conversion Pettigrew; Penalty Pettigrew. Wales: Tries G Wyatt 3, James 2, Jarvis 2, Appleyard, Boobyer; Conversion Jarvis 3; Penalty Jarvis.
Ontario: Pagano; Nichols, Hall, Armstrong, Cahoon; Pettigrew, Armstrong (Miller, 56); Jacques, Hendry, Ashley (Marshall, 67), Traynor, McCarthy, Mustgrave, Ross, Swindle.
WALES: M Back (Swansea); G Wyatt (Pontypridd), N Boobyer (Llanelli), J Lewis (Pontypridd), D James (Bridgend); L Jarvis (Cardiff), A Moore (Richmond, capt); I Buckett (Swansea), G Jenkins (Swansea), C Anthony (Swansea), S Moore (Swansea), N Watkins (Neath), C Wyatt (Llanelli), R Appleyard (Swansea), S Williams (Neath). Replacements: A Griffiths (Ponytpridd) for Buckett, 60; R McBryde (Llanelli) for Jenkins, 70.