The decorator got down on one knee on Christmas day, according to the Daily Mail, while the couple were holidaying in Bali.
After the 25-year-old singer's tragic miscarriage six months into her pregnancy in October, a source said that the engagement has given the pair something to smile about.
'It all came out of the blue, but Lily was overwhelmed and they are both overjoyed,' the insider said.
'Lily was telling everyone that she and Sam had just got engaged (and) they could not keep the smiles off their faces. It couldn't have been more romantic. The resort is just so peaceful and picturesque.'