Star Wars has revealed that Count Dooku was a member of the Jedi Council before turning to the dark side. Introduced as the face of the Separatist movement in Star Wars: Episode II - Attack of the Clones, Dooku (Christopher Lee) proved to be one of the most potent villains of the prequel era. In the last few years, Lucasfilm has released a range of novels dedicated to adding greater depth to this era, and Dooku's story has gradually been fleshed out.
The story of Count Dooku's fall to the dark side is told in Cavan Scott's Dooku: Jedi Lost. There's a sense that he fell for the same reason as Anakin Skywalker: because of his attachments — he stumbled upon his birth family and was drawn into the affairs of his homeworld of Serenno. There have been hints that this was orchestrated, or at least encouraged, by the Sith, potentially suggesting Darth Plagueis considered Dooku as an apprentice rather than Palpatine. The latest Star Wars anthology, Stories of Jedi & Sith, contains a short story by Alex Segura called "Resolve" that explores just how Dooku's fall affected Qui-Gon Jinn, who had served as Dooku's Padawan. The bond between a Jedi Master and an Apprentice is a sacred one, and when Dooku left the Jedi, he tore that bond apart, leaving Qui-Gon shaken to the core.
The new Star Wars novel Padawan, written by Kiersten White, reveals Count Dooku ascended higher into the ranks of the Jedi than was previously believed. During one conversation, a group of Padawans reflect on the effect of Dooku's decision to leave the Jedi, prompted by a visit he paid to the temple. "He's here often enough," one notes. "He still meets with the Council on occasion. Just because he's no longer on it..." The clear implication is that Dooku actually served as a member of the Jedi Council, although it's unclear whether he was a permanent fixture or whether he was rotated on to the Jedi Council on a provisional basis.
Star Wars' Jedi Council Reveal Makes Dooku's Dark Side Fall Even Bigger
It's fascinating to contrast Count Dooku's story in Disney canon to the one told in the old Expanded Universe, where he was offered a role on the Jedi Council but turned it down. This story is far more impactful, making Dooku a much more prominent figure among the Jedi — and, therefore, making his decision to leave them even more shocking. Star Wars has already established that Dooku attempted to persuade others to follow him out of the Jedi, and it's easy to see why he was able to do this: the Jedi would instinctively have deferred to him as an authority figure.
Padawan further reveals that Count Dooku continued to meet with the Jedi Council, offering them guidance and advice — presumably on political matters and the affairs of the Outer Rim, for Serenno was important there since the High Republic Era. This may have even continued into the prequel era, meaning Dooku was walking among the Jedi even as he trained in the ways of the Sith. No wonder Mace Windu insisted Dooku could be trusted in Star Wars: Episode II - Attack of the Clones, as he was still an adviser to the Jedi.