In 1912 James Lee decided to give up the hotel and once again became a miner. Jennie became a regular visitor to Mr. Garvie's Bookshop in Cowdenbeath. Garvie was blind and used to read to him from his favourite book, A History of the Working Classes. Another family friend, Jim Beveridge, lent Jennie a copy of the Ragged Trousered Philanthropist by Robert Tressell. Jennie Lee was also very fond of the Happy Prince by Oscar Wilde.

James Lee was chairman of the local branch of the Independent Labour Party. Jennie accompanied her father to these meetings and heard several leading socialists including James Maxton and David Kirkwood. Lee like most members of the ILP was opposed to Britain's involvement in the First World War. In the newspapers she read about Aneurin Bevan, a nineteen year old miner from South Wales, who refused to take part in the war. Bevan, who was later to marry Jennie Lee, told the local magistrates: "I am not and never have been a conscientious objector. I will fight, but I will choose my own enemy and my own battlefield and I won't have you do it for me."

As a fourteen year old schoolgirl, Jennie remembered the excitement of her father and friends when they heard news of the Russian Revolution. After the war James Lee was one of the leaders of the Hands off Russia movement that attempted to counterbalance the campaign led by Winston Churchill to invade Russia.

Jennie wanted to go to university but her parents were unable to afford the fees. However, with support from the Carnegie Trust, who agreed to pay half her fees and the Fife Education Authority, who awarded her a grant of £45 a year, she was able to become a student at Edinburgh University.

At university Jennie joined the Labour Club, the University Women's Union and the editorial board of the Rebel Student. One of Jennie's first campaigns was to have Bertrand Russell elected as Rector of the university. Russell a pacifist and campaigner for women's rights was a popular figure with university students at that time. During the First World War Russell was sacked from his post as a lecturer at Cambridge University and imprisoned for his involvement with the No-Conscription Fellowship (NCF), an organisation that encouraged men to refuse war service.

Jennie was a great reader and favourite authors at university included H. N. Brailsford, Bertrand Russell, H. G. Wells and Olive Schreiner. It was while at university in Edinburgh that Jennie gained her first experience of public speaking. Jennie could often be found on the Mound in Princess Street making speeches on Socialism and votes for women on the same terms as men. Jennie also attended weekend schools organised by the National Council of Labour Colleges where she met Ellen Wilkinson for the first time.

During the General Strike Jennie returned home to give support to the miners. She had just won a MacLaren Bursary, which she was able to hand over to her family struggling on her father's strike pay. When the strike was over James Lee, like many union activists, was sacked, blacklisted and after four months unemployment was forced to take work as a manual labourer.

Jennie's political work did not interfere with her studies and she left Edinburgh University with a degree in education and law. She taught at her school in Cowdenbeath but after an impressive speech at the 1927 National Conference of the Independent Labour Party, Jennie was invited to become the ILP candidate for North Lanark.

Jennie Lee was elected to Parliament at a by-election in February 1929 when she turned a 2,028 Conservative majority into a Labour majority of 6,578. At twenty-four she was the youngest member of the House of Commons. The Labour leadership selected Margaret Bondfield and the Chief Whip, Tom Kennedy, to introduce Jennie into Parliament. Jennie rejected the idea and insisted that two old friends from Scotland, Robert Smillie and James Maxton, should be her sponsors. This was the first of many conflicts that she was to have with the leadership of the party over the next few years.

Jennie's first speech in the House of Commons was a fierce attack on Winston Churchill and his budget proposals. Afterwards she congratulated Jennie on her speech and told her that he also wanted to help the poor but she had to understand that: "The richer the rich became, the more able they would be to help the poor".

In the House of Commons Jennie's closest friend was Frank Wise, one of the leaders of the Independent Labour Party. Wise was married and although he considered the possibility of divorce, they eventually decided against it. As she pointed out in a letter to Wise: "I feel that divorce and marriage would do both of us immense harm. Certainly public non-Catholic opinion is becoming more tolerant about divorce, but in triangles where all three parties are fairly equal as to age and other matters. The situation where a man has lived with one woman for twenty years, and becomes attached to another woman about twenty years younger, is too many unpleasant types, to be lightly accepted."

Another close friend was Aneurin Bevan, who represented Ebbw Vale in South Wales. Jennie was particularly impressed with Bevan's attack on David Lloyd George. The two young MPs had much in common. They both had fathers who were miners who had suffered terrible industrial defeats in 1919, 1921 and 1926. As she wrote later: "We were both now pinning our hopes on political action. We were eager to test to the full the possibility of bringing about basic socialist change by peaceful, constitutional means.

Jennie was totally opposed to Ramsay MacDonald and the National Government he formed in 1931. Like most Labour MPs who refused to support MacDonald, Jennie was defeated in the 1931 General Election. Over the next couple of years Jennie spent her time writing articles for the ILP New Leader and on lecture tours of the United States and Canada. In November 1933, Frank Wise collapsed and died. The following year she married Aneurin Bevan.

Jennie Lee was defeated again at North Lanark in the 1935 General Election. Jennie remained involved in politics, and was especially active in trying to persuade the British Labour movement to create a Popular Front with other European groups in an attempt to stop the spread of fascism in the 1930s.

In 1940 Lord Beaverbrook, Minister of Aircraft Production, employed Jennie Lee in his department. She later left to work as a journalist for the Daily Mirror. In the 1945 General Election, she won the mining constituency of Cannock in Staffordshire. In the government formed by Clement Attlee, Jennie's husband, Aneurin Bevan, was made Minister of Health and was responsible for the introduction of the National Health Service.

Jennie Lee agreed with Aneurin Bevan about most political issues, but was unhappy with his decision to reject unilateral nuclear disarmament at the 1957 Labour Party Conference. However, she was aware that his decision was based on what he thought was best for the Labour movement and was deeply shocked with the way he was treated by fellow socialists during and after his speech. For as he had told her, "I can just about save this Party, but I shall destroy myself in doing so."

After the 1964 General Election Lee was appointed arts minister and was responsible for what Harold Wilson later called the greatest achievement of his Labour Government, the setting up of the Open University. She retired from the House of Commons in 1970 when she was created Baroness Lee of Asheridge. Jennie Lee died in 1988.