He fought at the Battle of Karpenisi and completed the victory of his brother, the renowned Markos Botsaris.

In 1803 Kostas Botsaris and the remnants of the Souliotes crossed over to the Ionian Islands, where they ultimately took service in the French-raised Albanian Regiment.

[citation needed] In 1820, he fought to the end on Ali Pasha's side against the Ottoman army.

[4] On the night of 21 August 1823 Kostas, under the leadership of his brother Markos participated in the celebrated attack on Karpenisi by 350 Souliotes, against around 1000 Ottoman troops who formed the vanguard of the army with which Mustai Pasha was advancing to reinforce the besiegers.

Fifteen years after the death of his brother, the American traveller and author Mr. John Lloyd Stephens visited Kostas Botsaris, then a colonel in the service of King Otto of Greece in Missolonghi,[5] and described him as: A man of about fifty years of age, of middle height and spare build, who, immediately after the formal introduction, expressed his gratitude as a Greek for the services rendered his country by America; and added, with sparkling eye and flushed cheek, that when the Greek revolutionary flag sailed into the port of Napoli di Romania, among hundreds of vessels of all nations, an American captain was the first to recognize and salute it.Botsaris continued to serve in the Greek kingdom until his death in Athens on 13 November 1853.