Kelly graduated from Walter Johnson High School, and later attended the University of Wisconsin-Madison, where in 2002 she earned a Bachelor of Arts degree in history.

She was a member of the Health and Government Operations Committee, including as its vice chair from 2022 to 2023, during her entire tenure and served as deputy majority whip from 2016 to 2023.

Kelly also served as the president of the Women Legislators of Maryland caucus from 2017 to 2018, and as the House chair of the Joint Committee on Children, Youth, and Families from 2016 to 2022.

[17][18] In January 2019, Kelly was one of nine Maryland lawmakers to add their names to a manifesto signed by 326 state legislators to reaffirm their commitment to protecting abortion rights.

[22] In May 2022, following the leak of a draft majority opinion for the Supreme Court case Dobbs v. Jackson Women's Health Organization, Kelly joined 82 other legislators in signing a letter pushing Hogan to release $3.5 million in funding to support the implementation of the Abortion Care Access Act.

[23] In June 2022, following the Supreme Court's ruling in Dobbs, Kelly said that the end of Roe v. Wade made strong laws on the state level "even more necessary".

[25] In November 2023, Kelly and eight other state senators signed a joint letter that threatened to defund immigrants rights group CASA de Maryland because it had called for an immediate ceasefire in the 2023 Israel–Hamas war and condemned the "utilization of US tax dollars to promote the ongoing violence.

[4][27] In 2022, she introduced a bill that would provide paid family leave to all part- and full-time employees who have worked 680 hours in the past year.

[36] In 2013, Kelly voted in favor of a bill that would raise gas taxes in Maryland to fund the state's various mass transit projects.

[37] During the 2019 legislative session, Kelly introduced a bill to expand eligibility requirements for the state's child care tax credits.