After being in a long-term relationship with frequent collaborator and fellow poet Greta Bellamacina, the two married in July 2017. To celebrate Montgomery created a wedding fire poem which read, "SALVAGE PARADISE. LOVE IS THE WEATHER. DAYDREAMS FOREVER" before being set alight. Likewise, Bellamacina wrote him a fire poem reading "ETERNAL WINDOW HYMNS". The two met when Bellamacina was editing a book of love poetry and put one of Montgomery's poems in the book. She later said that "it was refreshing to meet someone through their words, you can find out a lot about the person." In 2015, Bellamacina gave birth to their first child, a son, Lorca.