The origins of the white papal garments are usually attributed to Pope St. Pius V (1566-72), as he was Dominican friar (who wore white) before becoming pope. Apparently, Pius V simply wanted to preserve his white Dominican cassock.
However, the first recorded mention of a pope wearing white actually occurs nearly 300 years before Pope St. Pius V.
In 1274, Pope Gregory X ordered published a document known as the Ordo XIII. a ritual book containing directions for liturgical functions, but not including the text of the prayers etc., recited by the celebrant or his assistants. These prayers were contained in separate books, e.g., the Sacramentary, Antiphonary, Psalter, but the Ordo concerned itself with the ceremonial.
In the Ordo, the ceremonies for the pope’s coronation were described in detail. Upon taking office, the pope was to wear a red cope, mozzetta, and shoes and a white cassock. The earliest portion of the document (nn. 1-12) is concerned with the choice, consecration, and coronation of a new pope, provision being made for the case of his being a bishop, priest, or deacon.
Actually, the Holy Father does not make use of the cassock. By tradition, he wears only the white simar on all occasions.
“Simar: The most familiar ecclesiastical dress of the Catholic hierarchy is the simar (or Zimarra), and yet this garment was not officially recognized as part of episcopal or clerical vesture until 1872, when it was granted status at the Papal Court as acceptable garb for papal audiences. Until the pontificate of Pope Pius IX, the simar was an optional garment available to all clergy, but reserved for use at home—thus the modern name “house cassock.” The simar has origins earlier than the late nineteenth century. Mention of it is made in apostolic briefs as early as 1760, but it did not become official vesture until 1872.
“The simar is often described as a cassock with shoulder cape, but it is not, strictly speaking, a cassock at all.[1] The hierarchy came to make use of the simar, which later came to replace the cassock for those elevated to the prelature, for wear outside the liturgical life of the Church. The shape of the simar is very similar to that of the cassock, but the simar has an elbow-length shoulder cape, which does not meet in the front, thus exposing the buttons from the collar to the toe. The simar traditionally included oversleeves of matching black wool, trimmed in the appropriate color of rank. These sleeves extended from the shoulder to the elbow and were opened in front and were closed by a row of five buttons, symbolizing the five wounds of Christ. The oversleeves were abolished by Pope Paul VI in 1969. This garment together with the cape and oversleeves, was known in the seventeenth and eighteenth centuries as the “pelerine.” Later the name zimarra, or simar, became more prominent.”[2]
[1] Secretariat of State Directive No. 135705.
[2] Noonan, The Church Visible, p. 292.