Paddy Cregg, in for the injured Murray Davidson, attempted a long-range effort that Paul Gallacher had to tip over the bar.
McDonald thought he had given the visitors the lead moments later when he met Dave Mackay's cross, but the offside flag denied the midfielder, while Wotherspoon's curling effort narrowly missed the target.
However, the Saints were to be denied no longer.
Erskine could not mark his Thistle return with a goal
Wotherspoon vindicated manager Tommy Wright's decision to introduce him to the starting line-up as his marauding run into the box allowed May to score his 16th league goal of the season.
That seemed to galvanise Alan Archibald's men into action as they came close through a series of free-kicks from Kallum Higginbotham and Lyle Taylor, while
on-loan Dundee United winger Erskine
, determined to impress back at Firhill, provided another dimension to the Jags.
And it was a much more positive in the second half for the hosts as Gary Fraser tested Stevie Banks from distance, while Taylor attempted a hook shot that flew wide.
Thistle were in the ascendancy and Lee Mair's shot almost caught Banks off guard but the goalkeeper reacted well to thwart the midfielder.
However, there was still that vulnerability to the counter for Partick and May, irrepressible at times, almost doubled their advantage when he flashed a header wide.
The Jags brought on Doolan and Christie Elliot in search of an equaliser in the second half, but it was Erskine who looked most likely to restore parity.
The winger attempted an audacious overhead kick that just went over, before following up with a left-foot effort that slipped wide.
And Doolan also came close late on inside the six-yard box, with his effort shaving the outside of the post.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.