McGinn, 27, gives the Ducks depth at forward, as Anaheim general manager Bob Murray filled out his forward group on Monday. In a separate deal, Murray acquired forward Brandon Pirri from the Florida Panthers for a sixth-round pick.
"I just thought we became deeper and more versatile," Murray said. "I'm comfortable with what we did without (messing) up the chemistry too much."
Murray declined to give up goalie Frederik Andersen or defenseman Sami Vatanen, retaining his soon-to-be restricted free agents to keep his roster core in position to challenge for the Stanley Cup this spring.
"I had lots of calls on certain people on the back end," Murray said, likely indicating interest in up-and-coming defensemen Shea Theodore and Brandon Montour. "I've got lots of thinking to do in the next four months ... but I wasn't going to give certain things to get players."
McGinn, who is in the final year of his contract, had 14 goals in 63 games with the Sabres this season. He was limited to 19 games last season because of back problems that required surgery. The two-way wing has 83 goals and 72 assists in 429 career NHL games over eight seasons with San Jose, Colorado and Buffalo.
"We know he can play in the West, let's put it that way," Murray said.
After scoring 22 goals in only 49 games last season, Pirri, 24, fell out of favor with the Panthers this season. He has 11 goals and 13 assists this season in 52 games and is a minus-4.
In the McGinn deal, if the Ducks win two playoff rounds, the third-round pick becomes a second-rounder.
In another deal, the Ducks traded Patrick Maroon to Edmonton for a fourth-round pick and Slovak defenseman Martin Gernat. The Ducks also swapped Tim Jackman for right wing Corey Tropp in a minor-league deal with Chicago.
Maroon will get a fresh start in Edmonton after struggling mightily this season for the Ducks. The power forward has just four goals and nine assists while putting up a team-worst minus-13 rating.
Murray called Maroon "the consummate team player." The forward is due $2 million in each of the next two seasons, clearing even more room in the Ducks' budget to re-sign restricted free agents Vatanen, Andersen and Hampus Lindholm.
The 22-year-old Gernat has three assists for the Oilers' AHL affiliate in Bakersfield this season.
Information from The Associated Press was used in this report.