The National Portrait Gallery, which has entertained presidents and politicians, threw a box lunch affair yesterday for some of the athletes included in its summer exhibit of sports art and memorabilia, "Champions of American Sport."

The luncheon, which preceded a late-afternoon White House reception for 600 and a black tie dinner and party for 400 to celebrate the show's opening today, was an opportunity for athletes and their families to make small talk among themselves, reporters and executives of the sponsors, Philip Morris Inc. and its subsidiary, the Miller Brewing Co.

Across tables decorated by bouquets planted in upside-down football helmets, Willie Mays' wife nibbled at herb-flavored popcorn and chatter with Jacke Robinson's widow. Well-rounded Red Auerbach, with his ever-present cigar, talked basketball with George Mikan, at 6 feet 10 the tallest man in the room. And despite the sophisticated company, rodeo cowboy Larry Mahan did not doff his silver-studded cowboy hat.

Mikan, Bob Cousy, Oscar Robertson and Auerbach, who is combining this visit to Washington with another luncheon tribute Wednesday at the Touchdown Club, were among the old-time basketball stars who turned up for lunch at the gallery.

"All of these acknowledgments get more significant with the years," said Cousy, who has remained in the basketball spotlight as the color man for Boston Celtics television broadcasts.

"When you're active you take these honors for granted. This honor has more impact for me, expecially because it involves the Smithsonian Institute," Cousy said.

Although Jesse Ownes' widow stayed in Arizona because of the anticipated air controllers' strike, two of Owens' daughters and three grandchildren came from Chicago.

Most of the attention went to Mays, who assumed the baseball spotlight after Hank Aaron and Babe Ruth's daughter, Mrs. Dominic Pirone, canceled their luncheon appearances.