Doctor Who: What Happened To Every Companion?
9. Tom Baker Era
Easily the most famous of the classic Doctors, Tom Baker's era exemplified all the best qualities of classic Who. And that includes his companions. Starting off with the irreplaceable Sarah Jane Smith.
Starting off as a companion of the 3rd Doctor, it's with Baker that the character truly came into her own. Having interacted with a total of 9 of The Doctor's incarnations, The Doctor saw Sarah not just as another companion or even a friend, but someone he could - and would always - trust. So when she left The Doctor as a regular companion, it was certainly sad. The Doctor had been called away to Gallifrey, and has to leave Sarah behind because The Time Lords despise outsiders on their planet. However, 4 being a scatterbrained sort, he accidentally dropped her in Scotland instead of South Croydon. They would interact here and there afterwards, the most notable of which being 10's era, when they reunite for the first time since The Time War.
Next is Hary Sullivan, the first person to interact with 4 seeing as how he was a doctor working with UNIT and had been keeping 3 stable until he regenerated. He only traveled with The Doctor on certain occasions, where he showed himself a capable and brilliant military doctor. After resigning from UNIT and bidding goodbye to The Doctor, he went on to NATO, MI5 and other organizations, continuing to help people.
Then we had Leela, a tribal woman from the planet Sevateem. Her dynamic with the Doctor was both hilarious and enlightening, seeing as how she's the one who FINALLY asks how the whole "bigger on the inside" thing works. She left when she and The Doctor had an adventure on Gallifrey, where she fell in love with a Time Lord named Andred. The Doctor - assuring Andred that Leela would look after him, left Leela on Gallifrey. Where - assuming the audio dramas and books aren't canon - she likely died in The Time War. Bummer.
This is also where K-9, the robot dog, departed from The Doctor's service, promising to keep an eye on Leela.
Finally we have Romana I and II. She was a Time Lord sent to help The Doctor look for the artifact known as The Key Of Time. Romana even regenerated during her travels with The Doctor, hence Romana 1 and 2. When the quest was done, she and The Doctor parted ways, where she went on to become Lady president of the Time Lords of Gallifrey.