Johannah Deakin, 43, died in the early hours of Wednesday morning (07Dec16) following a battle with a "very aggressive" form of leukaemia.
Louis' stepfather announced the sad news in a statement released on Friday (09Dec16), prompting the singer's famous friends to offer up their condolences on social media.
Former One Direction member Zayn Malik was among the first to reach out to his old pal via Twitter, and now Liam has done the same by posting a moving note online.
"Louis, I'm so sorry for you my brother my heart aches for you (sic)," it reads. "just know I love you the same from a million miles away as i do right next to you. I'm always here for you through everything as you have been for me.
"My thoughts and prayers are with you and your family at this incredibly sad time I can't begin to imagine what you are all going through (sic). Rip Johanna (sic) you are forever in my thoughts".
Liam's post emerged hours after his girlfriend Cheryl, who is reportedly expecting the couple's first child, tweeted a similar message to Louis.
Meanwhile, music mogul Simon Cowell, who helped to form One Direction on his U.K. TV talent show The X Factor, also shared his condolences on the microblogging site.
"Heartbroken for you @Louis_Tomlinson," he wrote. "Your mum Jay was an incredible person. Sending you all my love".
The sad news emerged as a surprise snippet of Louis' first solo track Just Hold On, produced by Steve Aoki, surfaced online, and the top dance DJ and producer subsequently reached out to the 24-year-old to express his sorrow at the tragedy.
"Everyone send love and strength to @Louis_Tomlinson and his family," Aoki posted on Twitter. "So so sorry my brother. We're all here for u (sic)."
Reports suggest Louis had been planning to debut the song with a surprise appearance on the live finale of Britain's The X Factor on Saturday (10Dec16). He is rumoured to be keeping the commitment, as per his mother's wishes, amid plans to dedicate the performance to Johannah.