Lowe also took to his Instagram to celebrate the occasion.
"✌️ years w my shadows best friend," he wrote.
And he's already got the stamp of approval from Lorde's mother, artist Sonja Yelich.
"2years 2day," she posted on Instagram as a caption to this pic of her daughter kissing her boyfriend on the cheek.
In 2014, Sonja addressed those criticizing the couple's 7-year age difference, telling Rolling Stone, "I didn't say, 'Yeah, sure, go date a 24-year-old.' But her dad and I met James and we liked him. When Ella was much younger, her first boyfriend was older -- four years or something."
Perhaps Lorde's longtime relationship shouldn't be surprising -- the GRAMMY-winning artist is fiercely loyal, especially when it comes to her friends. Check out the video below to hear how she dissed the size of producer Diplo's manhood when he insulted her bestie Taylor Swift on Twitter.