The on-loan Blackburn striker had only scored once in eight games since returning to United from the Lancashire club in the summer, but was the difference between the two sides on Wednesday evening.
He opened the scoring in the 33rd minute with a drive before heading home just before the break. By the time Goodwillie knocked in his third in stoppage time, United substitute Ryan Dow - on for Brian Graham - had added his name to the scoresheet for the hosts before Christie Elliott netted a consolation for the visitors.
McNamara said: "I never doubted he was capable of that. I'm delighted for him.
"I hope scoring three goals can be a turning point for him.
"The fourth goal, especially, was a great finish after a great bit of play from Ryan Gauld and a ball across at pace from Stuart Armstrong.
"There's been a lot of pressure on him to come up here and perform.
"Hopefully the other boys - like Nadir (Cifti), Armstrong and (Ryan) Gauld - chipping in with goals has taken a bit of weight off his shoulder and the goals will come. I hope there's more to come.
"At half-time I thought we were a bit fortunate to be two goals up as Partick Thistle caused us problems.
"But we carry a good threat, which showed in the end."
Jags boss Alan Archibald had no qualms about being undone by a big-money striker on loan from England.
He said: "No, that's the way football is. If we could afford him, we would take him.
"Goodwillie is clinical. He scored three goals and that's what that kind of money gets you.
"I don't know what he costs (in wages) but probably five or six of our players. And you see for yourself - he was the difference.
"But if your strikers aren't scoring you have to keep clean sheets and we made a couple of errors.
"Conrad (Balatoni) slipped for the first goal, which was disappointing because he had a good performance and we lost a goal from a set-piece.
"But apart from that we were the better team."