The Irish midfielder fired high into the net from 25 yards on the hour mark just as Motherwell looked to have gained control.
Michael Higdon had headed Motherwell into the lead three minutes after the break after twice being denied by goalmouth clearances in the opening half.
St Mirren midfielder Kenny McLean hit the bar in the first half and the visitors pressed for a winner in an end-to-end finale.
There were light patches of snow on the park but both teams defied the slippy surface as they attacked at every opportunity and played some slick football.
Goodwin's strike leaves Motherwell without a home win since September but they have still only lost to St Johnstone outside the Old Firm and are now eight points clear in third place. Darren Randolph spilled an early strike from David van Zanten but the Motherwell defence cleared and the home side began to get into their passing rhythm without being able to capitalise on some dangerous crosses.
The game burst to life in the 12th minute when Steve Jennings, angered that Goodwin's two-footed tackle had gone unpunished by referee Calum Murray, deliberately fouled Graham Carey in frustration.
The foul prompted a yellow card and a rebuke from Goodwin, who was also booked, and a wider confrontation, which also saw Shaun Hutchinson booked.
Murray was in danger of losing control but the game settled down before Motherwell had an escape after Hutchinson caught Jeroen Tesselaar with a sliding challenge just inside his box while trying to intercept a through ball.
Murray pointed to the spot but his assistant had flagged for offside for the initial pass.
Motherwell had an excellent chance on the half-hour mark when Jamie Murphy cut through the right-hand side of the Saints defence after being allowed to build up speed from the touchline as Marc McAusland waited in vain for a throw-in.
Murphy found himself one-on-one with goalkeeper Craig Samson but from a tight angle and he elected to cut the ball back for Michael Higdon eight yards out.
The ball kicked up and Higdon's effort was blocked by the away defence before Omar Daley sliced the rebound wide as he ran in beyond the far post.
Goodwin then cleared Higdon's off the line following Hateley's corner before St Mirren came closest to breaking the first-half deadlock when McLean's dipping 35-yard shot hit the bar.
Higdon finally netted against his former team in the 48th minute. Nicky Law used his pace and skill to get to the by-line and stood up a left-footed cross towards the back post where Higdon forced himself above David Barron to nod home.
Motherwell were enjoying the extra space as the visitors pushed men forward but they were brought crashing back down to earth when Goodwin pounced on Stephen Craigan's headed clearance and took a touch before unleashing a powerful strike into the roof of the net.
The Paisley side had a great chance to go ahead on the counter-attack within a minute when McLean's crossfield pass found Paul McGowan unmarked. The former Celtic player rounded Randolph but sliced wide as the angle narrowed.
St Mirren were lifted by their goal and flourished in possession as Motherwell struggled to push forward.
Motherwell brought on Chris Humphrey and Stephen Hughes for the impressive Daley and Keith Lasley, who had been booked, and soon turned the tide.
Humphrey cut inside two men and shot wide via a deflection and Murphy soon had a shot blocked by some desperate defending inside the box.
Humphrey could not find a killer cross as Motherwell continued to find him but Saints almost caught them on the break when McGowan played in substitute Aaron Mooy, making first appearance of the season following a back injury, but Randolph got down well to hold.
Randolph soon saved from Steven Thompson and Carey and then tipped over McLean's wonderful acrobatic effort.
Samson was called into action to beat away Murphy's powerful header following Hateley's cross and McLean missed a last-minute chance when he sliced over from 20 yards with his right foot.