"He was like a boy with a new bicycle," Toronto Telegram reporter Ron Collister is quoted as saying in the 1968 book "Assignment Ottawa" by his Telegram colleague Peter Dempson.
Yet Pearson's enthusiasm for the design — three red maple leaves on a white background bordered by two blue bars — resulted in hostilities between the Prime Minister's Office and members of the parliamentary press gallery, not to mention infighting among the journalists themselves.
About a dozen reporters were invited to enjoy a cup of tea with Pearson at 24 Sussex in July '64 and watch him unfurl his choice for the new flag, dubbed the Pearson Pennant.
As the reporters left, however, Pearson told them there was a catch to the visit — any stories about his pick for the design would have to be attributed to a "reliable source," not the prime minister himself.
Dempson's book recalls how Toronto Star Weekly reporter Walter Stewart, a relative newcomer to the press gallery, balked, uncomfortable with the idea of writing a positive story about Pearson's flag design without naming the source.
He subsequently identified Pearson as the source of the story and wrote of the visit to 24 Sussex, infuriating the prime minister to such an extent that instructions were issued to staff to deny Stewart any information in the future.
Stewart was subsequently treated by the PMO "with the caution one would accord a ticking mailbox," the late June Callwood wrote of the incident.
Other members of the press gallery who weren't invited to the flag shindig were miffed that the prime minister had played favourites. They also treated Stewart like a pariah for bucking press gallery convention and refusing to agree to Pearson's conditions.
"They knew I was the one who 'ratted,' and they did not like that one bit," Stewart recounted in the 1993 book "Scrum Wars" by Allan Levine.
As for Pearson's favoured flag, it was passed over for the now-iconic red Maple Leaf, which celebrates its 50th birthday on Sunday.
Follow Lee-Anne Goodman on Twitter @leeanne25
Note to readers: This is a corrected story. An earlier version had an incorrect publication date for "Scrum Wars"