It is possible that she became a member of the Order of the Phoenix in the First Wizarding War, although her most notable contribution to the cause was taking up residence in Little Whinging, most probably on Albus Dumbledore's orders, to keep an eye on young Harry Potter. This may have begun prior to the end of the First War, initially in order to watch over the Dursley family.
Arabella Figg lived in a Muggle community, working undercover for the Order of the Phoenix. Most of her job involved watching Harry Potter and ensuring that no harm came to him from Lord Voldemort. During times when the Dursley family took vacations, they left Harry Potter with Mrs. Figg, who once gave him an ancient chocolate cake as a "treat," but most often bored him senseless with pictures of her part-kneazle cats. She made sure he didn't particularly enjoy his time with her, and took care never to reveal her heritage to either Harry or the Dursleys, to ensure that the Dursleys would not withdraw Harry from her care.
In 1995, Harry and his cousin Dudley were attacked by dementors in Little Whinging. Figg sensed the distinct presence of the Dementors and despite, as a Squib, being unable to actually see them, she was still able to describe the Dementors in front of the Wizengamot during Harry's trial. Not trusting Mundungus Fletcher to effectively fulfill his duties of following and protecting Harry, she charged Mr. Tibbles with the task, and upon discovering Mundungus had abandoned his post, she eventually tracked Harry and his cousin Dudley down herself, to discover they had just barely survived a dementor attack. As the three walked back to the Dursleys, with Harry half carrying Dudley, she revealed that she was not a Muggle, as previously believed, but a Squib, and moreover, a member of the Order of the Phoenix. Before returning Harry to Privet Drive, she apologised for the way she had been forced to treat him while he was growing up.
Mrs. Figg was also a witness at Harry's trial before the Wizengamot, regarding his unauthorised use of magic as an underaged wizard. She arrived in her bedroom slippers, and her description of the Dementors was unimpressive, as she described them as "big and wearing cloaks." However, she ultimately managed to convince Amelia Bones and various other unbiased members of the Wizengamot with her accurate description of a Dementor's presence. Despite Cornelius Fudge's attempts to discredit her on the basis of being a Squib, Mrs. Figg, along with Albus Dumbledore, managed to persuade the Ministry to clear Harry of all charges.
During the Battle of the Astronomy Tower in 1997, Albus Dumbledore was killed by Severus Snape, who sided with the Death Eaters. Figg attended Dumbledore's funeral in 1997, at Hogwarts School of Witchcraft and Wizardry.
As she could not fight due to her lack of magic, it is unknown if she participated in the Battle of Hogwarts, though she may have assisted fighters in some non-magical capacity.
Arabella Figg is described as a batty old woman with grizzled grey fly away hair, wearing a hairnet and tartan carpet slippers.
In Harry Potter's youth, the Dursleys left him in Mrs. Figg's care at least once a year, and during their time together, she given to making him look endlessly at photos of "all the cats she'd ever owned". In 1995, she admitted that, though she didn't enjoy it, she had intentionally kept Harry bored and miserable, as the Dursleys would never have left him with her if they thought he enjoyed it. Though Harry only thought of her as his "his batty old cat-obsessed neighbour"before discovering she was a Squib, she appeared to be a clever woman with a bit of a temper. She physically attacked Mundungus Fletcher when he left his post of watching Harry Potter, and prior to that, had been quite sarcastic, even slightly insulting, during her conversation with Harry. She appeared nervous, even timid, during Harry's disciplinary hearing, though that was likely due to the stigma surrounding being a Squib, as well as the fact that she had to testify before the entire Wizengamot. Regardless, she was a brave, vigorous woman, who, despite her lack of magical ability, did her best to support the Order of the Phoenix.
Arabella Figg knew Albus Dumbledore at least since the time of the First Wizarding War. After Lord Voldemort's first downfall, Dumbledore trusted Figg with the task of watching Harry Potter during his time at the Dursleys' house. Figg appeared to respect Dumbledore and consider him a great wizard; when Harry was surprised that she knew Dumbledore, she replied that "everybody knows Dumbledore." Figg and Dumbledore both participated in Harry's trial before the Wizengamot in 1995 . She later attended Dumbledore's funeral at Hogwarts School of Witchcraft and Wizardry, in 1997.