In 1669, Mackay volunteered for the Venetian forces fighting in Crete; he rejoined the regiment and took part in the 1672 invasion of the Dutch Republic. Many Scots and English officers opposed supporting an attack by Catholic France against Dutch fellow Protestants. After his marriage in 1673, Mackay transferred to the Anglo-Scots Brigade, part of the Dutch military; he took part in the 1674 Battle of Seneffe and siege of Grave.