The expression mark my words means to advise someone to pay attention to what you were saying and remember the message because it is likely to be important or pre-empt something that will happen in the future. This expression usually follows serious advice or a caution.

1. Mark my words, Chris, you will regret stealing that money from me!
2. She will do really well in this sport, mark my words. I predict she will be professional in the next three years.

1. I am the right person to turn this company around and I will do. Mark my words.
2. She won't last a year in a job like that - you mark my words!