How had Noah found favor with the Lord?

Answer 1

The verse itself states explicitly, albeit generally:Noah found grace in the eyes of the Lord...Noah was, in his era, a man righteous and whole-hearted; Noah walked with God."

Meaning that he served God his whole life.

See also:

Evidence of the Flood

Answer 2

After describing how the whole world was ruined in the eyes of God because of how bad mankind had become, The Bible says that God found favor with at least one man on the Earth:

Genesis 6:8,9 (NWT which uses the divine name) - "But Noah found favor in the eyes of Jehovah. This is the history of Noah. Noah was a righteous man. He proved himself faultless among his contemporaries. Noah walked with the true God."

After instructing Noah to build the ark, the Bible then says this about Noah's response:

Genesis 6:22 "And Noah did according to all that God had commanded him. He did just so."

The Apostle Paul wrote this concerning Noah at Hebrews 11:7 (ESV) - "By faith Noah, being warned by God concerning events as yet unseen, in reverent fear constructed an ark for the saving of his household. By this he condemned the world and became an heir of the righteousness that comes by faith."

The Apostle Peter said this about Noah at 2 Peter 2:5 (NWT) - "And he (God) did not refrain from punishing an ancient world, but kept Noah, a preacher of righteousness, safe with seven others when he brought a flood upon a world of ungodly people."