A security guard received minor burns in the blaze as he tried to flee through the building's only exit.
She was eventually caught after being identified in CCTV footage from the office which picked up her distinctive coat and walking style.
According to The Independent, Fenwick was imprisoned for 20 months for her crimes.
She was sentenced for two counts of arson and one count of criminal damage at Harrow Crown Court after pleading guilty earlier this week.
"Fenwick's personal vendetta against her former employer led to her committing offences that not only caused a nuisance but also instilled a sense of fear among staff who became concerned about their safety at work," senior prosecutor Cynthia Caiquo said.
"Fenwick was identified using CCTV footage, which included her distinctive walking style and the jacket she was wearing, which was seized by police after a search of her home.
"The strong prosecution case against Fenwick meant that she had no choice but to plead guilty."
Caiquo said the court had also made an order restraining Fenwick from going anywhere near her former workplace.
"I hope this conviction and sentence gives employers the confidence they need to report crimes against them," she said.