How to put CR/LF into a TStringgrid cell?

TStringGrid uses Canvas.TextRect, which uses ExtTextOut, which in turn does not support drawing of multiline text.

You have to draw this yourself in an OnDrawCell event handler with WinAPI's DrawText routine. See for example this answer on how to use DrawText for multiline text, and this recent answer on how to implement custom drawing in OnDrawCell:

type
  TForm1 = class(TForm)
    StringGrid1: TStringGrid;
    procedure FormCreate(Sender: TObject);
    procedure StringGrid1DrawCell(Sender: TObject; ACol, ARow: Integer;
      Rect: TRect; State: TGridDrawState);
  private
    procedure FillWithRandomText(AGrid: TStringGrid);
    procedure UpdateRowHeights(AGrid: TStringGrid);
  end;

procedure TForm1.FillWithRandomText(AGrid: TStringGrid);
const
  S = 'This is a sample'#13#10'text that contains'#13#10'multiple lines.';
var
  X: Integer;
  Y: Integer;
begin
  for X := AGrid.FixedCols to AGrid.ColCount - 1 do
    for Y := AGrid.FixedRows to AGrid.RowCount - 1 do
      AGrid.Cells[X, Y] := Copy(S, 1, 8 + Random(Length(S) - 8));
  UpdateRowHeights(AGrid);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  FillWithRandomText(StringGrid1);
end;

procedure TForm1.StringGrid1DrawCell(Sender: TObject; ACol, ARow: Integer;
  Rect: TRect; State: TGridDrawState);
begin
  with TStringGrid(Sender) do
    if Pos(#13#10, Cells[ACol, ARow]) > 0 then
    begin
      Canvas.FillRect(Rect);
      Inc(Rect.Left, 2);
      Inc(Rect.Top, 2);
      DrawText(Canvas.Handle, PChar(Cells[ACol, ARow]), -1, Rect,
        DT_NOPREFIX or DT_WORDBREAK);
    end;
end;

procedure TForm1.UpdateRowHeights(AGrid: TStringGrid);
var
  Y: Integer;
  MaxHeight: Integer;
  X: Integer;
  R: TRect;
  TxtHeight: Integer;
begin
  for Y := AGrid.FixedRows to AGrid.RowCount - 1 do
  begin
    MaxHeight := AGrid.DefaultRowHeight - 4;
    for X := AGrid.FixedCols to AGrid.ColCount - 1 do
    begin
      R := Rect(0, 0, AGrid.ColWidths[X] - 4, 0);
      TxtHeight := DrawText(AGrid.Canvas.Handle, PChar(AGrid.Cells[X, Y]), -1,
        R, DT_WORDBREAK or DT_CALCRECT);
      if TxtHeight > MaxHeight then
        MaxHeight := TxtHeight;
    end;
    AGrid.RowHeights[Y] := MaxHeight + 4;
  end;
end;

---

There are also other StringGrid components able of drawing multiline text. For instance, this one which I wrote myself (download source: NLDStringGrid) with possibly this result:

var
  R: TRect;
begin
  NLDStringGrid1.Columns.Add;
  NLDStringGrid1.Columns.Add;
  NLDStringGrid1.Cells[1, 1] := 'Sample test'#13#10'Second line';
  NLDStringGrid1.Columns[1].MultiLine := True;
  NLDStringGrid1.AutoRowHeights := True;
  SetRect(R, 2, 2, 3, 3);
  NLDStringGrid1.MergeCells(TGridRect(R), True, True);
  NLDStringGrid1.ColWidths[2] := 40;
  NLDStringGrid1.Cells[2, 2] := 'Sample test'#13#10'Second line';
end;

The TStringGrid's default renderer don't support multiple lines. By setting the TStringGrid in OwnerDraw mode (by invoking the OnDrawCell event) you can render each cell by your own liking.

Have a look at this for an example that helped a previous user.

Linked reference code inserted:

procedure DrawSGCell(Sender : TObject; C, R : integer; Rect : TRect;
          Style : TFontStyles; Wrap : boolean; Just : TAlignment;
          CanEdit : boolean);
  { draws formatted contents in string grid cell at col C, row R;
    Style is a set of fsBold, fsItalic, fsUnderline and fsStrikeOut;
    Wrap invokes word wrap for the cell's text; Just is taLeftJustify,
    taRightJustify or taCenter; if CanEdit false, cell will be given 
    the background color of fixed cells; call this routine from 
    grid's DrawCell event }
var
  S        : string;
  DrawRect : TRect;
begin
  with (Sender as tStringGrid), Canvas do begin
    { erase earlier contents from default drawing }
    if (R >= FixedRows) and (C >= FixedCols) and CanEdit then
      Brush.Color:= Color
    else
      Brush.Color:= FixedColor;
    FillRect(Rect);
    { get cell contents }
    S:= Cells[C, R];
    if length(S) > 0 then begin
      case Just of
        taLeftJustify  : S:= ' ' + S;
        taRightJustify : S:= S + ' ';
        end;
      { set font style }
      Font.Style:= Style;
      { copy of cell rectangle for text sizing }
      DrawRect:= Rect;
      if Wrap then begin
        { get size of text rectangle in DrawRect, with word wrap }
        DrawText(Handle, PChar(S), length(S), DrawRect,
          dt_calcrect or dt_wordbreak or dt_center);
        if (DrawRect.Bottom - DrawRect.Top) > RowHeights[R] then begin
          { cell word-wraps; increase row height }
          RowHeights[R]:= DrawRect.Bottom - DrawRect.Top;
          SetGridHeight(Sender as tStringGrid);
          end
        else begin
          { cell doesn't word-wrap }
          DrawRect.Right:= Rect.Right;
          FillRect(DrawRect);
          case Just of
            taLeftJustify  : DrawText(Handle, PChar(S), length(S), DrawRect,
                               dt_wordbreak or dt_left);
            taCenter       : DrawText(Handle, PChar(S), length(S), DrawRect,
                               dt_wordbreak or dt_center);
            taRightJustify : DrawText(Handle, PChar(S), length(S), DrawRect,
                               dt_wordbreak or dt_right);
            end;
          end
        end
      else
        { no word wrap }
        case Just of
          taLeftJustify  : DrawText(Handle, PChar(S), length(S), DrawRect,
                             dt_singleline or dt_vcenter or dt_left);
          taCenter       : DrawText(Handle, PChar(S), length(S), DrawRect,
                             dt_singleline or dt_vcenter or dt_center);
          taRightJustify : DrawText(Handle, PChar(S), length(S), DrawRect,
                             dt_singleline or dt_vcenter or dt_right);
          end;
      { restore no font styles }
      Font.Style:= [];
      end;
    end;
end;

I think this will work fine for you...