Former Cheltenham midfielder Pack, 22, has agreed a two-year deal and has been training with City for the last week.
"Marlon Pack has been on our radar all summer and the boy and his agent have always made it clear that this was his preferred destination," manager Sean O'Driscoll told the club's official website.
"As a player who attracted rave reviews for Cheltenham, and has been in the League Two team of the year twice, it was no surprise he had a number of suitors.
"But the fact he was prepared to come in and train with us while we tried to work out the logistics of a move, and with no guarantees it would be definite, said a lot about his attitude and character."
Former West Ham striker Harewood, 33, has also penned a one-year deal after impressing at Ashton Gate in recent weeks.
O'Driscoll added: "Marlon has made a big impact with the boys since he's been here. Despite not even being a Bristol City player he has used his influence and experience really positively and set a fantastic example of how to be a good professional.
"Nothing is going to faze Marlon - he's been there and done it all before. That experience and calming influence in a young dressing room cannot be underestimated and he is a sensible addition that is not breaking the bank."
- Sports & Recreation