@Guest Hootenanny
For convenience I would describe Millar's career in 3 phases, theatrical/activist, radio career (radio ballads 1958 to 1964) Then the formation of the critics group in 1865.1964)
This is a bit of a gross simplification because folk plays an important role all the way through from the Kinderscout trespass of 1932
The Kinder mass trespass was a deliberate act of civil disobedience by men of the Young Communist League of Manchester, and others from Sheffield.The protest was intended to secure free access to England's mountains and moorlands. . A young man called James Henry Miller, better known as Ewan MacColl, was a keen rambler and an enthusiastic member of the Young Communist League. He played a major part in organising the publicity for the trespass, duplicating and handing out leaflets, though this role is disputed. He took part in the trespass, and was shocked by the violent reaction of the gamekeepers who met the ramblers on the hill, and the extremely harsh sentences handed down by the magistrates to the five ramblers who were arrested that day.What MacColl did not know was that the protest was to have a powerful long-term effect, leading to improved access to the countryside in the shape of national parks
The Manchester Rambler was written shortly after this event. He changed his name in 1945, 1949/50 different sources quote different dates. It is unimportant anyway - actors change their names frequently. Who makes a meal of Marion Michael Morrison masquerading as John Wayne?
When, in 1953 Theatre Workshop decided to move to Stratford, London, MacColl, who had opposed that move, left the company and changed the focus of his career from acting and playwriting to singing and ri
composing folk and topical song. His political activism was still strong during the miners strike(ArthurScargill presented him with a Davy Lamp as a thanks). You cannot really untangle his theatrical work from his politics and his song creation. His first record was released in 1950
I am trying to take no sides in this thread (I have a bias. I could not stand his singing or the finger in the ear sitting back to front on a chair. I do not profess to like all his songs, and his politics stank)
BUT: He created some stunning work, recorded some memorable albums, collaborated with the greats(such as Lomax, and revitalised British Folk Music, while composing a host of songs that most would readily label folk. That is quite some legacy that no detractors can deny.

p.s.There is a plaque dedicated to MacColl in Russell Square in London. The inscription includes: "Presented by his communist friends 25.1.1990 ... Folk Laureate – Singer – Dramatist – Marxist ... in recognition of strength and singleness of purpose of this fighter for Peace and Socialism".