The Doctor needed a new assistant after Liz Shaw departed, and thanks to an uncle, a high ranking civil servant, pulling some strings, Jo Grant was given the job. Despite the Brigadier having reservations about her not being a scientist, he hires her anyway, because he didn't have the heart to disappoint the poor girl.

An enthusiastic, bubbly and sometimes scatter-brained blond, Jo soon endears herself to the other members of UNIT, especially Captain Mike Yates and Sergeant Benton. The Doctor is also particularly attached to her, and she is devoted to him, refusing to leave his side even where mortal danger was involved.

jo is right there with the Doctor when the Time Lords restore his ability to travel in the TARDIS, and she joins him on his adventures with courage and determination. Although a bit naive in the beginning, Jo matures into a confident young woman able to stand on her own.

Because of that confidence, she is able to tell the Doctor goodbye at eh end of The Green Death when she decides to stay behind with Professor Clifford Jones, a young, Nobel Prize-winning scientist leading an environmentalist group, who she has fallen in love with. She agrees to marry Jones and go with him to the Amazon to study its vegetation, the news of which the Doctor greets with a mixture of pride and sadness.

Years later, Jo Grant (now Jo Jones) returned in Death of the Doctor meeting Sarah Jane Smith for the first time, and subsequently the Eleventh Doctor (whose regenerated appearance she accepts without requiring an explanation, having encountered his First and Second incarnations when Gallifrey was threatened by Omega).

While at a UNIT memorial service, Jo reveals to Sarah Jane that she is still married to Jones, has seven children and twelve grandchildren (with a thirteenth on the way), and spent her life after the Doctor travelling the world with her family campaigning for human rights and ecological issues. Her grandson, Santiago Jones, mentions at one point that Jo handcuffed herself to Robert Mugabe in one of her protests.

When Sarah tells her that she has met the Doctor several times since she left the TARDIS, Jo suggests he must have had more affection for her, as she personally never saw him again, despite her expectations. After Jo, Sarah Jane and the Doctor are teleported to The Wasteland of the Crimson Heart, Jo tells the Doctor that she tried to contact him while she and Professor Jones were traveling down the Amazon River, but was told by UNIT that he had left and had not come back.

When the Doctor talks with Jo, he assures her that he had not forgotten her in the intervening years, and had looked back on the lives of his companions the last time he was 'dying', with great pride, having simply never been able to visit her earlier as she spent so much time traveling that he was never able to find her.