McManus was born in North Melbourne, into a working-class family of Irish Catholic background.

[1] After Labor's defeat in the 1954 federal election, the federal Leader, Dr H. V. Evatt, publicly blamed the Victorian Branch and Santamaria's "Movement" for the defeat, causing a split in the Branch between pro- and anti-Evatt factions which eventually split the whole party.

[9] At the 1970 election, campaigning on the slogan "Vote Mac Back", he polled 19.1 percent, the DLP's best-ever result.

The DLP gave critical support to the Liberal governments of Robert Menzies and his successors, pressing them to adopt more militantly anti-communist policies both domestically and internationally, particularly on issues such as the Vietnam War and the recognition of the People's Republic of China believing there was a real threat from communist domination.

In 1974 the DLP supported the Liberal leader, Billy Snedden, in threatening to block the Whitlam government's budget bills in the Senate.

But this joint ticket did not eventuate, and all the DLP Senators lost their seats,[1] McManus polling only 6.4 percent in Victoria.