Waller followed his father by becoming a professional soldier and in 1617, he joined the army of the Venetian Republic, where he met the English mercenary leader, Sir Horace Vere. In 1620, he and Sir Ralph Hopton were members of the personal bodyguard for Elizabeth, Queen of Bohemia, sister of the future Charles I. After her husband Frederick was defeated at White Mountain in November, they escorted her to safety in Frankfurt.