Justin Morrow scored a hat-trick as Toronto FC sealed the Shield by accounting for New York Red Bulls 4-2 on Saturday.
Led by ex-Barcelona and Argentina boss Gerardo Martino, Atlanta made sure of a postseason berth with four regular-season games remaining.
Toronto FC were shocked 2-1 at New England Revolution as they were denied their first Supporters' Shield in MLS on Saturday.
Portland Timbers star Diego Valeri set an MLS record, and Atlanta United did likewise in front of a crowd of 70,425.
A brace from Jozy Altidore helped Toronto to a win over the San Jose Earthquakes, sealing a spot in the MLS play-offs.
Jack Harrison stepped up with an 84th-minute winner to help Patrick Vieira's New York extend their unbeaten streak to five matches.
Miguel Almiron's brace led Atlanta United past New York City in MLS.
Police confirmed 19 deaths following a blast at an Ariana Grande concert at Manchester Arena, which is being treated as a terror incident.
David Villa bagged a brace as New York City ended Orlando City's unbeaten streak at home in MLS with a 3-0 victory on Sunday.
Gerso Fernandes scored a 13-minute treble to inspire Sporting KC's 3-0 victory at home to Seattle Sounders on Wednesday.