Letitia "Tish" Jones was a PR executive. She was propelled into jobs well above her experience level because Harold Saxon was trying to draw her sister Martha Jones — and thus the Tenth Doctor — into his trap. To her own surprise, and her sister's mild suspicion, she was put in charge of the public events that launched Richard Lazarus' de-ageing machine. She later worked in Saxon's 10 Downing Street press office. In both capacities, she had significant interaction with the Doctor.
She then became the head of PR for Richard Lazarus, who made himself look 40 years younger. After the experiment he seduced Tish on the roof of his lab, just as he was about to transform into a lifeforce-draining monster.
Fortunately Martha and the Tenth Doctor arrived in time to save her. After everyone had evacuated the lab, Lazarus escaped to a cathedral where he chased Martha and Tish to the top of the tower and tried to kill them but the Doctor saved them by killing the mutated form of Lazarus with soundwaves. (TV: The Lazarus Experiment)
She then began to work for Harold Saxon when he became Prime Minister. The fact was he only hired her so that he could get to Martha.
When Martha, the Doctor and Jack Harkness went on the run, Tish, along with her mother and father, were captured and brought to the Valiant. There, they witnessed the Toclafane invasion of Earth. (TV: The Sound of Drums)
One year later, Tish was working on the Valiant as a slave with her mum and dad. She and her family tried to break out and stop the Master but they failed at each attempt. She and the rest of her family were present when Martha was brought onto the Valiant, and the Doctor used the psychic power of the entire world to restore himself and defeat the Master.
Following the destruction of the paradox machine, and subsequent reversal of time, Tish and the rest of the Jones family were among the few who remembered the events of the Year That Never Was. (TV: Last of the Time Lords)