"That was an important step for me, it's how you imagine your first day back will go," said Reus with a grin.
"I've worked hard for this moment and I am pleased that I could help the team."
The Germany winger led the way in a record-goal haul for a Champions League game.
The 12 goals in Dortmund on Tuesday bettered the competition's previous record of 11 set when Monaco beat Deportivo de La Coruna 8-3 in November 2003.
With Dortmund already through to the last 16, the huge win keeps them top of Group F, two points clear of Real Madrid with one match to go.
Dortmund's Shinji Kagawa and Legia's Aleksandar Prijovic finished with two goals each thanks to some poor defending by both teams.
Reus, who captained Dortmund, made his return after 185 days out with an adductor injury which sidelined him for June's European championships.
It was his first match since Dortmund lost the German Cup final to Bayern Munich on penalties in Berlin on May 21.
As is customary, the 27-year-old took the matchball home, but not before asking his team-mates to sign it.
"I'll let all the boys sign it and then it will have pride of place at home," he said.
Dortmund coach Thomas Tuchel was delighted with Reus' return as he welcomed back one of his key stars on a night when the goals flowed.
"You couldn't have expected him to score three times in his first game back," said Tuchel.
"This is extraordinary and a fantastic performance," he added in praising his team.
"This is the quality which has been missing for months and Marco has been sorely missed."
But Tuchel admitted Dortmund have clear defensive frailties to fix.
He said he was feeling sorry for goalkeeper Roman Weidenfeller, who was often left brutally exposed by his defence.
"It was surreal, I've never seen anything like it," said Tuchel.
"There is clearly room for improvement in terms of defence, especially on a counter-attack.
"I felt really sorry for Roman and we have to address these blatant errors."
Ex-Germany goalkeeper Weidenfeller, 36, back in the starting line-up after first-choice Roman Burki broke his hand in Saturday's 1-0 league win over Bayern Munich, was unhappy with his defence.
"There are points we should talk about, we must behave differently in defence," he fumed.
"We're playing here in the Champions League, you have to give a different account of yourself.
"It was a great game for the spectators, but we should not have leaked four."
Tuchel had made nine changes to the side which beat Bayern on Saturday and Prijovic took advantage of sloppy defence to chip his shot home and stun the Dortmund crowd on 10 minutes.
Even though UEFA had banned Warsaw's fans from this particular match for previous bad behaviour, a small band managed to get into the stadium and cheered their side's opener.
But Borussia roared back with five goals in 16 minutes.
Their first three goals came in 198 seconds when Kagawa headed home on 17 minutes, then slotted a second just a minute later.
To compound Warsaw's problems, a clearance punch from goalkeeper Radoslaw Cierzniak clattered off Nuri Sahin's chest and into the Legia goal to make it 3-1 on 20 minutes.
Prijovic pulled another goal back for Warsaw on 26 minutes but Dortmund's French winger Ousmane Dembele made it 4-2 on 29 minutes.Then came the first from Reus on 32 minutes and a second and Dortmund's sixth on 52 minutes.
Warsaw midfielder Michal Kucharczyk pulled a third goal back for Legia on 57 minutes before Dortmund defender Felix Passlack scored his first Champions League goal and Hungary striker Nemanja Nikolic netted Warsaw's fourth in the final ten minutes.
Reus wrapped up his hat-trick in the 93rd-minute when he converted Aubameyang's final pass at point-blank range.