Paul Padmore, of Kipling Road, Willenhall, admitted failing to meet the needs of Yorkshire terrier Poppy in front of Birmingham magistrates.
Padmore had not groomed or brushed Poppy the whole time he owned her, the RSPCA said.
The magistrates court was told the RSPCA became involved after a member of the public contacted the animal charity with concerns about Poppy's welfare.
When RSPCA inspector Beth Boyd visited the home, she said she was shocked by the state of 13-year-old Poppy.
Inspector Boyd said: "Poppy was completely covered in dreadlocks on every part of her body – she was even struggling to walk because of all the dreadlocks.
"Padmore admitted he had never brushed or groomed her the whole time he had her, which was years.
“Not only was her fur in a poor condition, but also her claws were overgrown.
"Initially she was quite aggressive but as soon as we got her to a vet her temperament changed completely and she became gentler to handle. She was clearly very uncomfortable.”
The dreadlocks were sheared off her at the vets, and in total the amount of fur which came off her was around 2kg.
Some of the dreadlocks were around 13.5 inches in length.
Inspector Boyd said: “This was purely long-term neglect.
“There is no excuse for a dog to be left in this condition. It would have made her quality of life virtually non-existent.”
Poppy was signed over into the RSPCA’s care and rehomed to a loving new owner.
But just weeks after moving to her new home, Poppy developed a serious spinal injury and had to be put to sleep.
“It’s sad that she died so soon after going to a new home, but she spent her last few weeks with a loving owner,” said Inspector Boyd.
As well as being disqualified from keeping animals for life, 58-year-old Padmore was given a six-week community order and ordered to pay costs of £100 and an £85 victim surcharge,