NI Election: Mid Ulster - O'Neill proves her identity, and then her vote-winning prowess
Mrs O'Neill did get into the Seven Towers Leisure Centre after party colleagues came to the rescue with her handbag and the all-important means of identification.
Later, the Co Tyrone native, who turned 40 in January, stormed across the finishing line by topping the poll with more than 10,000 votes.
Party colleagues Linda Dillon and Ian Milne soon followed in her wake.
Keith Buchanan (DUP) was elected with an increased vote, but admitted that it was "not a good day for unionism" in the constituency after Ulster Unionist Sandra Overend lost her seat.
She slipped away from the count centre long before it was announced that the SDLP's Patsy McGlone won the fifth seat without reaching the quota.
Later Mrs O'Neill insisted that she wanted to see Stormont working.
After grabbing 10,258 votes, she said: "If people are serious about making the institutions work, if people are genuine about power-sharing, about equality and respect, then whilst we have a difficult period ahead in the next few weeks, there is nothing that is unachievable if you have the right attitude.
"This has been a very good day for Sinn Fein. We said throughout the election: what we win is for all."
Mr Milne, who is from Bellaghy, said he was delighted the party had returned the full sweep of its candidates in Mid Ulster.
He added it was ready to work with the DUP.
"Absolutely, we are prepared to do business with anybody. That has always been the approach of Sinn Fein. We will talk with anyone and discuss and negotiate," he said.
Mr McGlone, meanwhile, implored them to work with each other, saying people had to "behave" and get along.
Mr Buchanan said, on a personal level, he was happy with his result.
He said his bigger than expected vote showed the party was still getting good support and added: "RHI came up exactly seven times (on the doorsteps) to me."
Mr Buchanan had the second highest first preference vote in Mid Ulster with 9,568, which he said was partly down to the DUP being out on the ground early in the run-up to the election when others were tucking into turkey.
"We even canvassed over the Christmas holidays, that proved to be successful," he said.
"I am very impressed and delighted. We were expecting just over 8,000 votes, but it went in excess of that."
Results Centre - every count as it happened: North Antrim - East Antrim - South Antrim - North Belfast - East Belfast - South Belfast - West Belfast - Strangford - South Down - Lagan Valley - Upper Bann - Newry and Armagh - Fermanagh & South Tyrone - West Tyrone - Mid Ulster - East Londonderry - Foyle - North Down