The letter ⟨m⟩ is silent at the start of a word before ⟨n⟩. This pattern is rare — it really only appears in mnemonic and a few related technical words.
Greek origin: ⟨mn-⟩ comes from Greek words about memory (Mnemosyne was the Greek goddess of memory). In English, only the ⟨n⟩ is pronounced, so "mnemonic" sounds like "nemonic".