MacManus joined the De Leonist Socialist Labour Party (SLP)[1] and began work at Singers in Clydebank, then known as part of the Red Clydeside.

[2] Supporting the SLP's opposition to World War I, MacManus was arrested in 1915 at a meeting in George Square, Glasgow, for speaking against the threatened introduction of conscription.

[3] MacManus became a leading member of the Clyde Workers Committee, and for supporting David Kirkwood in the William Beardmore and Company strike of 1916, he was one of five people deported to Edinburgh.

[6] MacManus became its colonial secretary,[7] and attended the Fourth Congress of the Comintern in September,[8] at which he was elected to its Executive Committee and Praesidium.

This forgery, intended to damage the Labour Party's chances in the 1924 general election, was purportedly signed both by Grigory Zinoviev and MacManus.