Andrew originally lived in London with his brother Arthur. He and Arthur were very close, their relationship comparable to parabatai, and the pair fought best side by side. At one point, the brothers went to Idris to attend the Shadowhunter Academy.
There, Andrew saw Nerissa, a lady of the Seelie Court, and was so thunderstruck by her beauty that he pledged himself to her. He quickly decided to stay in the Faerie. The Shadowhunters believed that Andrew had accidentally ingested faerie juice, which then bound him to the Faerie and the Queen. Arthur insisted on staying with him. Apparently, the Academy teacher bargained with the Queen, and they agreed that their imprisonment would last only a day—a deal the faeries upheld with trickery: the brothers were kept in the land of the fey for what passed as only one day in the human world, but lasted around seven years in Faerie and to the brothers.
Andrew and Nerissa became lovers—possibly married in fae standards as, because she was a Downworlder, their marriage would not recognized by the Clave. Together, they had two children: Helen and Mark. Nerissa gave Andrew a silver chain as a token of her love—a choker that the Shadowhunters would later say was enchanted to make Andrew fall in love with Nerissa—and she taught him the ways of the Fair Folk.
During his time with Nerissa, he heard screams and cries of agony that belonged to his brother, who was under the control of Nerissa's sister, but he was led to believe that it was some animal. Eventually, Andrew found out the truth. Nerissa, worried that Andrew would go mad with grief and guilt, made new memories for him—false memories that would make it seem as if he was there against his will, that it would have been impossible to do anything to help Arthur.
Nerissa then set up their escape by releasing Arthur, who then attacked her to rescue Andrew, and Nerissa let them believe that she had been killed. When they returned, they reported everything they believed happened to the Inquisitor. The brothers left Idris and parted ways afterwards, becoming slightly estranged. Arthur went to London, and Andrew went to Los Angeles, where he met and fell in love with Eleanor, one of the Shadowhunters training at the Institute.
Eventually, Andrew married Eleanor and had their first child, Julian. Shortly after, Nerissa left their two children on the doorstep of the Institute. Andrew and Eleanor took in his half-fey children and raised them as Shadowhunters. Andrew never told Helen and Mark about what happened in the Faerie—or at least what he believed happened—but he subtly told them through constant readings of the John Keats poem La Belle Dame Sans Merci.
When word began to spread through the Shadowhunter community about Sebastian Morgenstern and the Infernal Cup, the friend of his children, Emma Carstairs, overheard Andrew say he would rather die than be Turned into one of Sebastian's Endarkened warriors.
However, when the Institute was unexpectedly attacked by Sebastian and his Endarkened forces, Andrew, along with every other present Conclave member, was forced to drink from Cup. Held down by Amatis, he was Turned, and, as a result of drinking from the demonically-aligned Cup, lost his own will in the process and fell under Sebastian's control. Afterwards, Andrew was the one who restrained his children's tutor, Katerina, while Sebastian forced her to drink from the Cup. Sebastian also made Andrew restrain his own son, Mark, whom he needed to be contained, though not Turned, for delivery to the faeries in exchange for their help.
Andrew participated in the fight with his fellow Endarkened at the Citadel battle. During the fight in Alicante, Andrew was among the Dark Shadowhunters who broke into the Accords Hall. He went directly for his children and tried to reach for Ty, in an attempt to coax him into coming with him. However, his cold tone convinced Emma and Jules that Andrew was really no longer himself. Before he could hurt Ty or the others, Jules threw a knife at his chest and killed him, angering and devastating his younger brother. Andrew was mourned by his children and his brother, Arthur.
Andrew had the Blackthorn trademark curling brown hair and blue-green eyes. His son, Julian, strongly resembled him.
- Pale Kings and Princes (mentioned and appeared in flashback)
- The Red Scrolls of Magic (mentioned only)
- City of Heavenly Fire (first appearance)
- Bitter of Tongue (mentioned only)
- Angels Twice Descending (mentioned only)
- Lady Midnight (mentioned and appeared in flashback)
- Lord of Shadows (mentioned only)
- Queen of Air and Darkness (mentioned only)
- The Lost World (mentioned only)
- Secrets of Blackthorn Hall (mentioned only)
- Andrew was a classicist, deeply fascinated by Greek and Roman history and mythology, and thus named all of his children after Greek and Latin historical figures.
- Andrew once had a British accent that he has since lost.
- Andrew was originally named Arthur, before he was renamed, and the name Arthur was then given to the character of Andrew's brother, instead.