The Lady Black Cats went into the match with a two-point advantage over the second-placed Belles, but with an inferior goal difference and knew that only victory would do at the Den.
But goals from Kelly McDougall, Rachel Furness, Keira Ramshaw and Abbey Joice ensured Sunderland achieved promotion to WSL 1, subject to meeting off-field licensing requirements.
Watch highlights of Millwall 0-4 Sunderland
"I’m feeling elated, it’s fantastic," said Sunderland boss Claire Robinson.
"All credit to the girls, I dedicate it to them and I couldn’t have asked anything more of them. It’s been a long season for them and to do it on the last day is fantastic.
"To be honest, I didn’t really look at the Doncaster score, it was all about what we were doing. We needed the second goal to make it comfortable.
"All four goals were fantastic, but the second one stood out as it was a building block to move the game on and we were more or less in control after that.
"We didn’t come here expecting to win, we knew that we had to work hard to get that result and the girls showed their quality.
"I attribute our success to the players. They’ve done the hard work. Everyone in the staff and the board have played their part in it as well."
Doncaster did wrack up their seventh win in a row and biggest victory margin of the season, as Courtney Sweetman-Kirk and Millie Bright scored two goals apiece, while Beth England, Jess Sigsworth, Sue Smith, Rhiannon Roberts all got on the scoresheet and Anneka Nuttall turned into her own net.
Reading finished third with a 3-0 win over Watford, where Bianca Bragg scored twice and Shelly Cox was also on target.
Fourth placed Villa struck twice late on to beat Oxford 2-0, with Maddy Cusack and Maisy Baker on target there.
Meanwhile, Durham and Yeovil shared the spoils in a 1-1 draw, as Ellie Christon gave Durham the lead with her first goal for the club, before Sarah Wiltshire equalised.