Harriet Jones was an unassuming, lifelong backbench MP, serving the (fictional) constituency of Flydale North, who happened to be at 10 Downing Street when an alien spacecraft crash-landed in the River Thames at the beginning of Aliens of London. She was there to discuss plans for the Flydale Infirmary in her "tiny little" constituency. She soon discovered that the Cabinet had been infiltrated, and that the Acting Prime Minister, Joseph Green, was actually an alien in disguise.

When the Doctor showed up at Downing Street to consult on the crash, she tried to warn his companion Rose Tyler of what was happening, but the two were pursued by the unmasked aliens. Eventually becoming trapped together with the Doctor and Rose in the Cabinet Rooms, she aided the Doctor in determining the Slitheen's weaknesses (World War Three). Finally, as the only elected representative present, she gave the Doctor the order to launch a Harpoon missile at Downing Street, killing the Slitheen before they could trigger off a nuclear war (Jones, the Doctor and Rose survived thanks to the reinforced steel walls of the Cabinet Rooms). At story's end, the Doctor told Rose that Jones was destined to be a future Prime Minister, elected for three successive terms, and the architect of Britain's Golden Age.

As portrayed by actress Penelope Wilton, for whom writer and producer Russell T. Davies specially wrote the character, Jones is a modest and intelligent middle-aged woman who had served her community faithfully for years without any real ambition beyond that. However, she was able to adapt quickly to the reality of the Slitheen threat, and took to the role of leadership easily, proceeding to reassure the public at the end that the crisis was over.

Harriet Jones returned in The Christmas Invasion, where she had become Prime Minister since the events of World War Three. Some of her acts as Prime Minister included the launching the Guinivere Space Probe by the British Rocket Group and extending a Cottage Hospital scheme nationwide. It was the space programme that led to her being faced with an alien invasion by the Sycorax. Although the invasion was averted by the Tenth Doctor, Jones ordered the covert Torchwood organisation to shoot down the retreating Sycorax ship (a reference to Margaret Thatcher's controversial decision to sink the Argentinian cruiser, the ARA General Belgrano during the 1982 Falklands War).

Furious with Jones, the Doctor warned her he could bring her down with just six words, which he whispered to her aide: "Don't you think she looks tired?" Soon she was fending off rumours of ill-health and was facing a vote of no confidence. How this plays into the Ninth Doctor's prediction that she would be elected for three terms is not clear.

A running gag associated with Jones in Aliens of London and World War Three was that she always introduced herself by flashing her identity card and saying, "Harriet Jones, MP, Flydale North." She explained that this was because her constituency is a "tiny little place" and people would not know about it. In The Christmas Invasion, despite being Prime Minister, she continued to flash her card and say, "Harriet Jones, Prime Minister," to which people (including the Sycorax leader) would invariably respond that they knew who she was.

As is common with the portrayal of fictional politicians on British television, Harriet Jones's political party is not stated. However, her line from Aliens of London about being "hardly one of the babes" gives one clue. This is a reference to the large number of female Labour Party MPs who entered the House of Commons in Labour's 1997 general election victory, who were dubbed "Blair's Babes" by the British media. This suggests, albeit not conclusively, that she is a Labour Party politician.