Tommy Wright's Saints travel to Norway for a Europa League second round qualifier on Thursday, and were approaching this fixture in a serious manner as a consequence.
Their intent was underlined by ex-Ton midfielder Chris Millar, who was a man possessed in the opening stages and went close to scoring when he crashed a shot against the near post.
At the other end, Weatherson, who played the first 20 minutes of the match, came within a whisker of marking the occasion with a goal when he glanced a Dougie Imrie cross narrowly wide.
In the second half, Archie Campbell and new signing Kabba-Modou Cham drew fine saves from visiting keeper Alan Mannus, while Rory Fallon tested Nicolas Caraux with a rising drive.
The deadlock was finally broken on 74 minutes when Saints full-back Dave Mackay placed a precision free-kick in the postage stamp top corner.
Weatherson's younger brothers, Paul and Daryl, were given a late run out and almost added a fairy-tale ending when Daryl crashed an angled drive against the outside of the post in the last minute.
Despite the result, it was a fitting - and emotional - send-off for a player who gave Ton 10 years of sterling service.
Morton (4-2-3-1): Caraux; Taggart, Page, McLaughlin, Fitzpatrick; Hands (Wallace, 61), Bachirou; Imrie (Campbell, 61), McKee (D. Weatherson, 87), O'Brien (Crawford, 70); Peter Weatherson (Cham, 20) (Paul Weatherson, 87).
Subs not used: Jamieson, Russell, Gaston (gk), Knight, Verlaque.
St Johnstone (4-3-3): Mannus; Mackay, Miller, Wright, Scobbie; Millar, Cregg (May, 46), McDonald (Caddis, 83); Wotherspoon (Edwards, 65), McLean (Fallon, 46), Hasselbaink (B. Easton, 65).
Subs not used: Banks (gk), Rodger, Kane, D. Easton.
Referee: Bobby Madden