Use cummax
df.High.cummax()
0 954
1 954
2 954
3 955
4 956
5 956
6 956
7 956
Name: High, dtype: int64
df['Max'] = df.High.cummax()
df
Compute the running (cumulative) maximum for a series in pandas
Use cummax
df.High.cummax()
0 954
1 954
2 954
3 955
4 956
5 956
6 956
7 956
Name: High, dtype: int64
df['Max'] = df.High.cummax()
df