Origins
Pattern and patron are twins separated at birth. Both descend from Latin patrōnus — 'protector, defender' — from pater, 'father'. In medieval usage, a patron was someone who provided a model: a father figure whose example others followed. That 'model' sense became literal. A patron was a template — a shape cut from paper or wood that a tailor used to cut cloth.
For centuries, English used the same word for both the benefactor and the template. A patron of the arts and a dressmaking patron were spelled and pronounced identically. Only in the 17th century did the language split them: patron kept the 'supporter' meaning, while the altered pronunciation pattern took the 'template' meaning.
The further shift from 'template' to 'repeated design' was natural. If you use a template repeatedly, a pattern emerges. By the 18th century, pattern meant any regular arrangement — wallpaper patterns, weather patterns, behaviour patterns.
Later History
French never made this split. The word patron still means both 'boss' and 'pattern' (as in a sewing pattern). Spanish patrón works the same way.
The Latin root pater produced one of the largest word families in English: paternal, patriot, patrimony, patronise, patriarch, and now pattern. All carry the shadow of fatherhood — authority, protection, and the models set by those who came before.