With the Liberal Party battling to hang on to his old seat, Mr Howard urged voters in the northern Sydney electorate to throw their support behind Mr Alexander, who had "worked his heart out for the people of Bennelong".
Mr Alexander, meanwhile, seemed happy to play second fiddle and let the former PM do most of the talking.
"When the great man is speaking, I'm very happy to listen, because whenever he speaks, I learn," Mr Alexander said.
Mr Howard, who served as member for Bennelong for 33 years, still appears to strike a chord with voters in the electorate, with shoppers and passers by jostling for selfies with the former PM.
John Kollias, who has owned a chemist in Eastwood for 19 years said that like many small business owners in the area, he associates the Howard years with a strong economy. "He's still a great campaigner", Mr Kollias said.
With former NSW Labor premier Kristina Keneally catching up in the polls, Howard's appearance provided a welcome dose of star power for the Liberals.
"We're not taking this election lying down," Howard said. "It's a byelection in challenging circumstances for the government.
"You've got the resources of the Labor Party, GetUp! and the unions, and a candidate for the Labor Party who's got a high profile, although she carries a lot of political baggage because of her failures as the premier of New South Wales."
Mr Alexander resigned from Parliament last month after British authorities failed to uncover evidence that he was not a British citizen.
Mr Howard praised Mr Alexander's "upfront" response to the citizenship affair, but said it was "unbelievable" that the former tennis star had been ineligible in the first place.
"If there ever was a dinky-di Aussie, it's John Alexander," Mr Howard said.