In 1874, he won a construction contract with the Chespeake and Ohio Railroad, using former contacts from his Service in the Civil War. By 1892, when Nancy was thirteen years old, her father had re-established his wealth and built a sizeable home. Chiswell Langhorne later moved his family to an estate, known as Mirador, in Albemarle County, Virginia.

The couple were well matched, as they were both American expatriates with similar temperaments. They were of the same age, born the same day, 19 May 1879. Astor shared some of Nancy's moral attitudes, and had a heart condition that may have contributed to his restraint.

Nancy Langhorne had four sisters and three brothers who survived childhood. All of the sisters were known for their beauty; Nancy and her sister Irene both attended a finishing school in New York City. There Nancy met her first husband, socialite Robert Gould Shaw II, a first cousin of Colonel Robert Gould Shaw, who commanded the 54th Massachusetts Regiment, the first unit in the Union Army to be composed of African Americans. They married in New York City on 27 October 1897, when she was 18.

The marriage was unhappy. Shaw's friends said Nancy became puritanical and rigid after marriage; her friends said that Shaw was an abusive alcoholic. During their four-year marriage, they had one son, Robert Gould Shaw III (called Bobby). Nancy left Shaw numerous times during their marriage, the first during their honeymoon. In 1903, Nancy's mother died; at that time, Nancy Shaw gained a divorce and moved back to Mirador to try to run her father's household, but was unsuccessful.

Nancy Shaw took a tour of England and fell in love with the country. Since she had been so happy there, her father suggested that she move to England. Seeing she was reluctant, her father said this was also her mother's wish; he suggested she take her younger sister Phyllis. Nancy and Phyllis moved together to England in 1905. Their older sister Irene had married the Artist Charles Dana Gibson and became a model for his Gibson Girls.

Viscountess Astor was not the first woman elected to the Westminster Parliament. That was achieved by Constance Markievicz, who was the first woman MP elected to Westminster in 1918, but as she was an Irish Republican, she did not take her seat. As a result, Lady Astor is sometimes erroneously referred to as the first woman elected to Parliament rather than the first woman to take her seat in Parliament.

Astor appealed to voters on the basis of her earlier work with the Canadian Soldiers, allies of the British, other charitable work during the war, her financial resources for the campaign and her ability to improvise. Her audiences appreciated her wit and ability to turn the tables on hecklers. Once a man asked her what the Astors had done for him and she responded with, "Why, Charlie, you know," and later had a picture taken with him. This informal style baffled yet amused the British public. She rallied the supporters of the current government, moderated her Prohibition views, and used women's meetings to gain the support of female voters. A by-election was held on 28 November 1919, and she took up her seat in the House on 1 December as a Unionist (also known as "Tory") Member of Parliament.

During the 1920s, Astor made several effective speeches in Parliament, and gained support for her Intoxicating Liquor (Sale to Persons under 18) Bill (nicknamed "Lady Astor's Bill"), raising the legal age for consuming alcohol in a public house from 14 to 18. Her wealth and persona also brought attention to women who were serving in government. She worked to recruit women into the civil Service, the police force, education reform, and the House of Lords. She remained popular in her constituency and well liked in the United States during the 1920s, but her success is generally believed to have declined in the following decades. She was also concerned about the treatment of Juvenile victims of crime: "The work of new MPs, such as Nancy Astor, led to a Departmental Committee on Sexual Offences Against Young People, which reported in 1925."

Nancy Astor's accomplishments in the House of Commons were relatively minor. She never held a position with much influence, and never any post of ministerial rank, although her time in Commons saw four Conservative Prime Ministers in office. The Duchess of Atholl (elected to Parliament in 1923, four years after Lady Astor) rose to higher levels in the Tory Party before Astor did. Astor felt if she had more position in the party, she would be less free to criticise her party's government. She did gain passage of a bill to increase the legal drinking age to eighteen unless the minor has parental approval.

Despite having Catholic friends such as Belloc for a time, Astor's religious views included a strong vein of Anti-Catholicism. Christopher Sykes argues that Kerr, an ex-Catholic, influenced this, but others argue that Astor's Protestant Virginia origins are a sufficient explanation for her Anti-Catholic views. (Anti-Catholicism was also tied into historic national rivalries.) In 1927 she reportedly told James Louis Garvin that if he hired a Catholic, "bishops would be there within a week."

The 1930s were a decade of personal and professional difficulty for Lady Astor. In 1928 she won a narrow victory over the Labour candidate. In 1931 Bobby Shaw, her son from her first marriage, was arrested for homosexual offences. As her son had previously shown tendencies toward alcoholism and instability, Astor's friend Philip Kerr, 11th Marquess of Lothian, suggested the arrest might act as a catalyst for him to change his behavior, but he was incorrect.

Astor made friends among other women MPs, including members of the other parties. Margaret Wintringham was elected after Astor had been in office for two years. Astor also befriended "Red Ellen" Wilkinson, a former Communist then a member in the Labour Party. Astor later proposed creating a "Women's Party", but the female Labour MPs opposed this, as their party was in power and had promised them positions. Over time, political differences separated the women MPs; by 1931 Astor became hostile to female Labour members such as Susan Lawrence.

The period from 1937 to the end of the war was personally difficult for her: from 1937–38 Astor lost both her sister Phyllis and her only surviving brother. In 1940 Lord Lothian died. He had been her closest Christian Scientist friend even after her husband converted. George Bernard Shaw’s wife died three years later. During the war, Astor's husband had a heart attack. After this, their marriage grew cold, likely due to her subsequent discomfort with his health problems. She ran a hospital for Canadian Soldiers as she had during the First World War, but openly expressed a preference for the earlier Soldiers.

When war did come, Astor admitted that she had made mistakes, and voted against Chamberlain, but hostility to her politics remained. In a 1939 speech, another MP, Stafford Cripps, called her "The Member for Berlin".

Her fear of Catholics increased and she made a speech saying that a Catholic conspiracy was subverting the foreign office. Based on her opposition to Communists, she insulted Stalin's role (from 1941) as an ally of the Western nations during the war. Her speeches became rambling and incomprehensible; an opponent said that debating her had become "like playing squash with a dish of scrambled eggs". On one occasion she accosted a young American soldier outside the Houses of Parliament. "Would you like to go in?" she asked. The GI replied: "You are the sort of woman my mother told me to avoid".

Lady Astor did not acknowledge her loss of popularity. She believed her party and her husband caused her retirement in 1945. As the Tories believed she had become a political liability in the final years of World War II, her husband said that if she ran for office again the family would not support her. She conceded but, according to contemporary reports, was both irritated and angry about this.

It was generally believed that it was Lady Astor who, during a World War II speech, first referred to the men of the 8th Army who were fighting in the Italian campaign as the "D-Day Dodgers". Observers thought she was suggesting they were avoiding the "real war" in France and the Future invasion. The Allied Soldiers in Italy were so incensed that Major Hamish Henderson of the 51st Highland Division composed a bitingly sarcastic song to the tune of the popular German song "Lili Marleen" (popularised in English by Marlene Dietrich), called "The Ballad of the D-Day Dodgers". This song has also been attributed to Lance-Sergeant Harry Pynn of the Tank Rescue Section, 19 Army Fire Brigade.

Lady Astor struggled in retirement, which put more strain on her marriage. In a speech commemorating her 25 years in parliament, she stated that her retirement was forced on her and that it should please the men of Britain. The couple began travelling separately and soon were living apart. Lord Astor also began moving toward left-wing politics in his last years, and that exacerbated their differences. However, the couple reconciled before his death on 30 September 1952.

After 1956 Nancy Astor became increasingly isolated. In 1959 she was honored by receiving the Freedom of City of Plymouth. By this time, she had lost all her sisters and brothers, her colleague "Red Ellen" Wilkinson died in 1947, George Bernard Shaw died in 1950, and she did not take well to widowhood. Her son Bobby Shaw became increasingly combative and after her death he committed suicide. Her son Jakie married a prominent Catholic woman, which hurt his relationship with his mother. She and her other children became estranged. Gradually she began to accept Catholics as friends. But, she said that her final years were lonely.

She disliked Jews and discouraged the hiring of Jews or Catholics to positions at The Observer. This regime persisted at The Observer into the 1960s and 70s. When Kenneth Tynan resigned from the Observer in 1963 to take the post of Literary Manager of the National Theatre, he proposed the Irish Playwright Dominic Behan as his preferred replacement. Behan, a well-known atheist, was interviewed for the post. When he told the interview board that he was a Catholic, the offer was withdrawn. Behan later said to Tynan, "I would hope if a Nazi ever asked me my religion I would have the courage to defend my right to be a Jew, Bush Baptist or even a damn Catholic!"

Lady Astor died in 1964 at her daughter Nancy Astor's home at Grimsthorpe Castle in Lincolnshire. She was cremated and her ashes interred at the Octagon Temple at Cliveden.

After the marriage, the Astor couple moved into Cliveden, a lavish estate in Buckinghamshire on the River Thames that was a wedding gift from Astor's father. Nancy Astor developed as a prominent hostess for the social elite. The Astors also owned a grand London house, No. 4 St. James's Square, which is now the premises of the Naval & Military Club. A blue plaque unveiled in 1987 commemorates Astor at St. James's Square. Through her many social connections, Lady Astor became involved in a political circle called Milner's Kindergarten. Considered liberal in their age, the group advocated unity and equality among English-speaking people and a continuance or expansion of British imperialism.

Astor was hampered in the popular campaign for her published and at times vocal teetotalism and her ignorance of current political issues. Her tendency to say odd or outlandish things sometimes made her appear unstable. On one occasion, while canvassing in Plymouth, she was greeted at a door by a girl whose mother was away. As Astor was unfamiliar with the area, she had been given a naval officer as an escort. The girl said: "...but she [my mother] said if a lady comes with a Sailor they're to use the upstairs room and leave ten bob". This is equivalent to £22 in 2016.

She was noted for exchanges with Winston Churchill, though these are not well documented. Churchill is supposed to have told Lady Astor that having a woman in Parliament was like having one intrude on him in the bathroom, to which she retorted, "You’re not handsome enough to have such fears." Lady Astor is also said to have responded to a question from Churchill about what disguise he should wear to a masquerade ball by saying, "Why don't you come sober, Prime Minister?" In another recounted exchange, Lady Astor said to Churchill, "If you were my husband, I'd poison your tea," to which he responded, "Madam, if you were my wife, I'd drink it." The retort has been documented as being by Churchill's friend F. E. Smith, Lord Birkenhead.