McAllister created her JennXPenn channel on January 15, 2009, but had experience with YouTube already, through a channel called "KikoAndPenn" she created with her best friend at the time, Coco. They did sketch videos and music videos, in hopes of becoming "the female Smosh". McAllister gained many subscribers early on in her YouTube career when she joined the collab group MyCollab in late 2009. MyCollab was started by YouTuber supermac18, and other members included ohitsdalton, mutantboyfriend (Jonah Green), TCProductionsX (Tyler, now itstylercx but no longer active),and imdaviddalen (no longer active). MyCollab ended after issues arose between supermac18 and Dalton. In May 2014, McAllister went on tour throughout the US and parts of Canada with friend, fellow YouTuber, and musician Tyler Ward. McAllister not only held paid meet and greets before Ward's concert, she also hosted the concert. The tour lasted about a month and was very successful for both McAllister and Ward. In March 2015, she announced that she would be releasing a book through Scholastic Publishing, called "Really Professional Internet Person". It is an autobiography, chronicling her entire life. The book was released on August 25, 2015.
In June 2012, McAllister started working with a YouTube channel called "AwesomenessTV". She initially uploaded two videos for the channel, a sketch where she goes camping in her backyard and a video about her summer bucket list for 2012 (on her main channel, JennXPenn). In August 2013, she became the host of a new show called "JennXPenn's Top 10s", a show where she says the top 10 of a single topic. ("Top 10 Pet Peeves", "Top 10 YouTube Videos of 2013, etc.) On January 20th, 2014, the "Jenn and Andrea/Andrea and Jenn Show" premiered on AwesomenessTV's channel. The show followed the lives of McAllister and her friend/roommate Andrea Russett (GETTOxFABxFOREVER) as they adjusted to life in Los Angeles and being roommates. The show ended in June of that year, due to McAllister and Russett moving into a house with fellow YouTubers Arden Rose and Lauren Elizabeth. As of now, McAllister is currently partnered with the AwesomenessTV network.
Throughout the spring and early summer of 2013, McAllister worked with Teen.com on YouTube doing a public prank video series called "Stranger Danger". She worked alongside Joey Graceffa and strawburry17 (Meghan).
McAllister has been the asked the question "How did you come up with your username?" on numerous occasions. She responded in an "Ask Jenn", "Well, Jenn is obviously my name", and then goes on to explain the "Penn" part. The story goes that she received the nickname around 5th- 6th grade from her friend. She says that when she went to create her YouTube channel, the username "JennPenn" was taken, so she simply added an "X" in between "Jenn" and "Penn", making "JennXPenn". In July 2013, McAllister moved to Los Angeles, California from a town near Philadelphia. She was staying in a house with fellow YouTubers Andrea Russett (GETTOxFABxFOREVER), Arden Rose, and Lauren Elizabeth. In 2015, she is living with YouTuber and best friend Lauren Elizabeth Luthringshausen.
Up until her senior year, McAllister attended public school in Pennsylvania. However, after moving out to California, she began online schooling. It was rumored that she dropped out of high school, after her announcement to stay in Los Angeles for the foreseeable future. She squashed that rumor in her vlog "HIGH SCHOOL DROPOUT". McAllister says that she was a good student, never failing a class and being part of a few Honors level courses. As for college, McAllister stated in her video "18 ON 18", "One thing I definitely want to do is go to college and get a degree, whether it's this year or later on in my life." In Really Professional Internet Person, she stated that she would be interested in pursuing a business degree.
In September 2012, McAllister's three YouTube channels were hacked and deleted.
Jenn has never mentioned the incident in any of her videos, but reluctantly opened up about it in Really Professional Internet Person, stating that it was "a really upsetting and terrifying period in [her] life.", but that it is "ultimately part of [her] story."
During the hack, she took a break from social media, out of fear that the hackers would attack again. McAllister was a victim of "doxxing". Her hacker posted a document containing all sorts of private information, including her phone number as well as her parents' phone numbers, her former address in Pennsylvania as well as her father's address in New Jersey, and even the passwords to all of her online accounts. They also deleted her ICloud account erasing her contacts, photos, and more. This information was obtained through a method of hacking in which a virus tracked every keystroke on McAllister's computer. The doxxers took it as far as to send a pink dildo to McAllister's house that, she remarked in her book, Really Professional Internet Person, resembled a, "pink alien penis." Jenn and her mother constantly hid the object around the house, making Jenn, "thankful for the dildo," which is also quoted from her book.
Jenn and her parents took the case to the Philadelphia Police Department. They searched for clues, and once they found the hacker commit credit card fraud using McAllister's mother's credit card, they were able to take the case to the FBI and bring the group responsible to justice. During this period, McAllister's parents kept in contact with law enforcement and sheltered her from some of the darker aspects of this incident, such as "lewd" and "threatening" messages left on their answering machine, in an effort to not worry their daughter any more than she already was. Jenn felt unsafe in her own home, having nightmares in which she ran around her home trying to lock all of the doors and windows before a "strange man" got in. Law enforcement officials had even prohibited McAllister from leaving her house through the front door.
Soon enough, she was able to get her three channels and all of her videos back.
Jenn has publicly stated on Twitter that she does not have a Kik or Ask.fm, however several impersonators exist on those networks. Jenn asks that you report any impersonators immediately.