"We played the majority of the football, they got one shot off and it found the back of the net.
"Just simple finishing things - Quinn should have scored a header but it hit the top of Scott [Burney]'s head and pushed it wide.
"I don't know if their keeper grew his fingernails out, but he managed to push them over the net."
Thistle had less chances, but made them count as they pulled away in the second half.
"Symon [King] said, 'we could have played till Christmas and we still wouldn't have scored," said Jason King.
Players were distraught and annoyed with themselves at fulltime, before King had them take stock of how far they have come this year after a disastrous 2016 season where they finished with just five points at the distant bottom of the table.
"It just shows how much higher they have set the bar for themselves."
As well as beating Rangers in two out of three meetings, Athletic finished with the second best goal tally and second least goals conceded behind the dominant champions AFC Havelock North.
Ryan Holden's 17 goals for the campaign was second overall behind Ranger's Sam De St Croix with 19.
The Athletic 1sts will have a week's break before they try to lift the Charity Cup off The Big Orange Wanganui City 1sts on September 9.
Meanwhile, the GVCA Wanganui Athletic Reserves provided some consolation for the weekend as they won their last Horizons Premiership home game against wooden-spooners Marist Development 4-2 at Wembley.
Sergio Serra scored a hat trick for the home side, which made four in two games, with Samuel Conley getting the other goal.
The visitors goals came from Jonathan Block and Blair Lankshear.
Reserves will play their rain-postponed match against Levin AFC at Donnelly Park to wrap up their Premiership season this Saturday.